Re: [PATCH net-next-2.6] netlink: netlink_recvmsg() fix

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 21 juillet 2010 à 10:05 +0200, Johannes Berg a écrit :
> On Tue, 2010-07-20 at 17:20 +0200, Eric Dumazet wrote:
> 
> > [PATCH net-next-2.6 v2] netlink: netlink_recvmsg() fix
> > 
> > commit 1dacc76d0014 
> > (net/compat/wext: send different messages to compat tasks)
> > introduced a race condition on netlink, in case MSG_PEEK is used.
> > 
> > An skb given by skb_recv_datagram() might be shared, we must copy it
> > before any modification, or risk fatal corruption.
> 
> Makes sense to me, seeing that if you MSG_PEEK it just increases
> skb->users. But nothing could touch the other skb at the same time?
> Although I guess with netlink multicast we have a similar situation.

Nothing can touch this skb at the same time but us and our friends
(consumers that did a skb_recv_datagram( MSG_PEEK ) operation).

Oh well, I see skb_unshare() tests skb_cloned(). This is not what we
want.

We probably wants something like :

if (skb_shared(skb)) {
	nsbk = skb_copy(skb, GFP_KERNEL);
	...
}